In recent years, demand for higher reliability battery systems for onboard vehicle use has increased, and in particular it has become desirable to enhance the reliability of battery systems that use, for example, lithium cells that store high amounts of power. Battery systems have been proposed that include improved diagnosis functions for heightening their reliability. For example a battery system for onboard vehicle use is disclosed in Patent Document #1 that, in order to enhance reliability, is equipped with a diagnosis circuit that diagnoses whether or not any anomalous condition is occurring in the battery system.